This Summer, Sotheby's and Christie's will be bringing works by British post-war greats Lucian Freud and Francis Bacon to the saleroom.

Two self-portraits by Bacon, which have not been in the public eye for a number of years, are to come out of their private collection to be sold at Sotheby's, London. The works are expected to cause a bidding frenzy and reach £15 million each.

Bacon produced the portraits around 40 years ago, Bacon experts did not know who purchased the painting but knew of its existence.

Francis Bacon, Three Studies for a Self-Portrait, 1980Image via Sotheby's

The works portray the artist in his 70s as he became obsessed with his own mortality. Bacon was known to have dyed his hair and worn make-up in his 60s in order to mask his ageing. In the portraits Bacon paints himself favourably, his way of internally and externally defying his inevitable ageing.

Sotheby's senior international specialist in contemporary art, Oliver Barker, said the public unveiling of the piece was "a pretty extraordinary collecting moment."

"We knew of the existence of the paintings but simply had no idea where they could be."

"Marlborough Fine Art kept a photographic archive and so both of these paintings appeared in a book on Bacon's self-portraits but, apart from being reproduced in books, they've not been seen."

"We knew of the existence of the paintings but simply had no idea where they could be.The first time I saw these paintings it was such a wonderful awakening. They're both so luminous."

"Bacon throughout his life was really questioning human existence. Never is that more pivotal in his work than in his self-portraits."

The work will go on sale at Sotheby's London Contemporary Art Evening Sale on 1st July.

On 13th May Christie's will auction Lucian Freud's masterpiece Big Sue, from the artist's Benefits Supervisor series. In 2008 Benefits Supervisor Sleeping, 1995 sold for £22 million to infamous billionaire Roman Abramovich.

Benefits Supervisor Resting, 1994, will go on sale in New York in May and is estimated to reach between £20-33 million in the saleroom. The series of four features Sue Tilley, a local government officer who Freud began to paint in 1993.

This version of Tilley has come the collection of Boston philanthropist Barbara Lee who purchased it from Acquavella Galleries in New York in 1995. The gallery was selling Freud's works to enable him to pay off his gambling debts.

The work has a guaranteed minimum price, which has not been revealed. This will be the first work of Freud's which has an estimate over £20 million since his death in 1988 and is part of the increase in sales of abstract works.